This invention relates to a semiconductor light emitting element, semiconductor light emitting device, image display device, and so on. More specifically, the invention relates to a semiconductor light emitting element, semiconductor light emitting device, image display device, and any other elements and devices configured to prevent external leakage of primary light emitted from a light emitting layer and to thereby waveform-convert it into secondary light and extract it with a remarkably high efficiency.
Semiconductor light emitting elements and various types of semiconductor light emitting devices using same have various advantages, such as compactness, low power consumption and high reliability, and are used in progressively wider applications, such as indoor and outdoor display panels, railway and traffic signals, car-borne signal illuminators, which are required to be highly luminous and highly reliable.
Among these semiconductor light emitting elements, those using gallium nitride compound semiconductors are being remarked recently. Gallium nitride compound semiconductors are direct-transitional III-V compound semiconductors which can efficiently emit light in relatively short wavelength ranges.
Throughout the present application, the xe2x80x9cgallium nitride compound semiconductorxe2x80x9d pertain to III-V compound semiconductors expressed by BxInyAlzGa(lxe2x88x92xxe2x88x92yxe2x88x92z)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61) and to any mixed crystal which includes phosphorus (P) or arsenic (As), for example, as group V species in addition to N in the above-mentioned chemical formula.
Gallium nitride compound semiconductors are remarked as hopeful materials of LEDs (light emitting diodes) and semiconductor lasers because the band gap can be changed from 1.89 to 6.2 eV by controlling the mole fractions x, y and z in the above-mentioned chemical formula. If highly luminous emission is realized in short wavelength ranges of blue and ultraviolet, the recording capacities of all kinds of optical discs will be doubled, and full color images will be realized on display devices. Under such and other prospects, short wavelength light emitting elements using gallium nitride compound semiconductors are under rapid developments toward improvements in their initial characteristics and reliability.
Structures of conventional light emitting elements using gallium nitride compound semiconductors are disclosed in, for example, Jpn. J. Appl. Phys., 28 (1989) p.L2112; Jpn. J. Appl. Phys., 32(1993) p.L8; and Japanese Patent Laid-Open Publication No. 5-291621.
FIG. 141 is a cross-sectional view schematically showing a conventional semiconductor light emitting element. The semiconductor light emitting element 6100 shown here is a gallium nitride semiconductor light emitting element. The light emitting element 6100 has a multi-layered structure of semiconductors stacked on a sapphire substrate 6120, namely, a buffer layer 6140, n-type contact layer 6160, n-type cladding layer 6118, light emitting layer 6120, p-type cladding layer 6122 and p-type contact layer 6124 which are stacked in this order on the sapphire substrate 6120.
The buffer layer 6140 may be made of n-type GaN, for example. The n-type contact layer 6160 has a high n-type carrier concentration to ensure ohmic contact with the n-side electrode 6134, and its material may be GaN, for example. The n-type cladding layer 6118 and the p-type cladding layer 6122 function to confine carriers within the light emitting layer 6120, and their refractive index must be lower than that of the light emitting layer 6120. The light emitting layer 6120 is a layer in which emission occurs due to recombination of electric charges injected as a current into the light emitting element.
The light emitting layer 6120 may be made of undoped InGaN, for example, and the cladding layers 6118 and 6122 may be made of AlGaN having a larger band gap than the light emitting layer 6120. The p-type contact layer 6124 has a high p-type carrier concentration to ensure ohmic contact with the p-side electrode 6126, and its material may be GaN, for example.
Stacked on the p-type contact layer 6124 is the p-side electrode 6126 which is transparent to the emitted light. Stacked on the n-type contact layer 6160 is the n-side electrode 6134. Bonding pads 6132 of Au are stacked on these electrodes, respectively, so that wires (not shown) for supplying a operating current to the element be bonded. The surface of the element is covered by the protective films 6130 and 6145 of silicon oxide, for example.
The conventional light emitting element 6100 is so configured that light emitted from the light emitting layer be directly extracted externally, and involved the problems indicated below.
One of the problems lies in variable emission wavelengths caused by structural varieties of light emitting elements. That is, semiconductor light emitting elements, even when manufactured under the same conditions, are liable to vary in quantity of impurities and in thicknesses of respective layers, which results in variety in emission wavelength.
Another problem lies in changes in emission wavelength depending upon the operating current. That is, emission wavelength of a semiconductor light emitting element may change depending upon the quantity of electric current supplied thereto, and it was difficult to control the emission luminance and emission wavelength independently.
Another problem lies in changes in emission wavelength depending upon the temperature. That is, when the temperature of a semiconductor light emitting element, particularly of its light emitting layer, changes, the effective band gap of the light emitting layer also changes, and causes an instablility of the emission wavelength.
As explained above, in conventional semiconductor light emitting elements, it was difficult to entirely control varieties in structure, temperature and electric current and to thereby limit changes in emission wavelength within a predetermined range, such as several nm(nanometers).
Conventional semiconductor light emitting devices involved an additional problem, namely, materials and structures of semiconductor light emitting elements used therein had to be determined and changed appropriately in accordance with desired emission wavelengths, such as selecting AlGaAs materials for emission of red light, GaAsP or InGaAlP materials for yellow light, GaP or InGaAlP materials for green light and InGaN materials for blue light.
It is therefore an object of the invention to provide a semiconductor light emitting element and a semiconductor light emitting device which are highly stable in emission wavelength and can wavelength-convert light with a high conversion efficiency in a wide wavelength range from visible light to infrared band.
According to the first aspect of the invention, there is provided a semiconductor light emitting element and a light emitting device comprising a wavelength converter located adjacent to a light extraction end of the light emitting layer to absorb the primary light emitted from the light emitting layer and to release secondary light of a second wavelength different from the first wavelength.
The first aspect of the present invention is embodied in the above-mentioned mode, and attains the effects explained below.
Light from the light emitting layer is not extracted directly but converted in wavelength by a fluorescent material. Therefore, it is prevented that the emission wavelength varies with varieties of manufacturing parameters of the semiconductor light emitting elements, drive current, temperature and other inevitable factors. That is, the invention realizes remarkable stability of emission wavelengths and makes it possible to control the emission luminance and the emission wavelength independently.
The fluorescent material may include a plurality of different materials to obtain a plurality of different emission wavelengths. For example, by appropriately mixing different fluorescent materials for red (R), green (G) and blue (B) to form the fluorescent material in each light emitting element, emission of white light can be obtained easily.
The material and the structure of the semiconductor light emitting elements used in a device need not be changed depending on the desired emission wavelength of the device. That is, in conventional techniques, optimum materials had to be selected to form semiconductor light emitting elements in accordance with desired emission wavelengths, such as selecting AlGaAs materials for emission of red light, GaP materials for yellow light, InGaAlP materials for green light and InGaN materials for blue light. However, according to the invention, it is sufficient to select appropriate fluorescent materials, and the material of the semiconductor light emitting element need not be changed.
Even when a device needs an arrangement of a plurality of semiconductor light emitting elements having different emission colors, such elements for different emission colors can be made only by changing the material of the fluorescent member, and all of the semiconductor light emitting elements may be common in materials and structure. This contributes to simplification of the structure of the light emitting device, remarkable reduction of the manufacturing cost and higher reliability. Additionally, by uniforming the drive current, supplied voltage or the size of the elements, its application can be extended remarkably.
According to the second aspect of the invention, there is provided a semiconductor light emitting element, a light emitting device and a image display device comprising a light emitting layer for emitting primary light of a first wavelength, a wavelength converter located adjacent to a light extraction end of the light emitting layer to absorb the primary light emitted from the light emitting layer and to release secondary light of a second wavelength different from the first wavelength, and a first optical reflector located adjacent to a light release end of the wavelength converter and having a lower reflectance for the secondary light released from the wavelength converter and a higher reflectance for the primary light passing through the wavelength converter.
Since the optical reflector RE1 is provided, the primary light having leaked through the wavelength converter FL can be reflected with a high efficiency and can be returned back to the wavelength converter FL. The primary light returned back in this manner is wavelength-converted by the wavelength converter FL, and passes through the optical reflector RE1 as secondary light. That is, the optical reflector RE1 located adjacent to the emission end of the wavelength converter FL prevents leakage of primary light by returning part of the primary light passing through the wavelength converter FL back to it for wavelength conversion thereby. Therefore, the primary light can be wavelength-converted with a high efficiency. Additionally, the wavelength converter FL is prevented from being exited by outer turbulent light and from emitting undesired light.
The semiconductor light emitting element may include an optical absorber AB. In this case, the optical absorber absorbs primary light passing through the optical reflector RE1 and prevents external leakage thereof. The light absorber AB also functions to adjust the spectrum of the extracted light and to improve the chromatic pureness. Additionally, since the light absorber AB absorbs ultraviolet rays entering from the exterior, it is prevented that such external turbulent light undesirably excites the wavelength converter FL and causes undesired emission.
The semiconductor light emitting element may further include a reflector RE2 to reflect primary light back into the wavelength converter FL. As a result, primary light can be wavelength-converted and extracted with a higher efficiency.
The semiconductor light emitting element may further includes an optical reflector RE3 for greater improvement of the wavelength conversion efficiency. In this case, not only the primary light but also the secondary light or any other optical component different in wavelength from the primary light can be prevented from external leakage. The optical reflector RE3 has a limitative aperture so that light can exit only through the aperture. By decreasing the size of the aperture, a light emitting element as a point-sized light source can be made easily. Such a point-sized light source enables effective collection of light by lenses or other optical elements, and it is therefore practically advantageous in most cases.
The semiconductor light emitting element may further include an optical reflector RE4 to more efficiently extract secondary light by reflecting it after wavelength conversion by the wavelength converter FL.
According to the invention, it is also possible to realize an image display device with a low power consumption, long life, high reliability, quick rising and good mechanical reliability.
As explained above, the invention provides a semiconductor light emitting element, semiconductor light emitting device and image display device which are simple in structure, stable in emission wavelength, excellent in emission efficiency, and capable of highly luminous emission in a wide wavelength range from visible light to infrared bands, and the invention promises great industrial contribution.
Moreover, the invention can provide various applications of the semiconductor light emitting element or device, such as illuminators, which are more efficient, lower in power consumption and longer in life than conventional fluorescent lamps and bulbs.
The illuminator according to the invention comprises: a semiconductor light emitting element for emitting ultraviolet rays; and a fluorescent element for absorbing said ultraviolet rays emitted from said semiconductor light emitting element and for releasing secondary light having a longer wavelength than said ultraviolet rays.
Said semiconductor light emitting element preferably contains gallium nitride compound semiconductor in a light emitting layer thereof.
Preferably, said secondary light is substantially a visible light.
Preferably, a predetermined number of said semiconductor light emitting elements are serially connected to form a unit, and a plurality of said units are connected in parallel.
The illuminator preferably further comprises a converter circuit for converting a high frequency voltage into a d.c voltage so that said semiconductor light emitting elements be driven when connected to a power source of a fluorescent lamp.
The illuminator preferably further comprises a first optical reflection film located between said semiconductor light emitting element and said fluorescent element, and having a wavelength selectivity to pass said ultraviolet rays and to reflect said secondary light released from said fluorescent element.
The illuminator preferably further comprises a second optical reflection film located on one side of said fluorescent element opposite from said semiconductor light emitting element, and having a wavelength selectivity to reflect said ultraviolet rays and to pass said secondary light released from said fluorescent element.
The illuminator preferably further comprises a light absorber located on one side of said fluorescent element opposite from said semiconductor light emitting element, and having a wavelength selectivity to absorb said ultraviolet rays and to pass said secondary light released from said fluorescent element.
The illuminator preferably comprises a wiring board; light emitting devices supported on said wiring board; and a translucent outer shell encapsulating said wiring board, each said semiconductor light emitting device including: said semiconductor light emitting element; and said fluorescent element.
The illuminator preferably comprises a wiring board; a plurality of semiconductor light emitting elements supported on said wiring boards; and a translucent outer shell encapsulating said wiring board, said outer shell having a fluorescent element on the inner wall surface thereof.
The illuminator preferably further comprises a pulse generator for supplying a pulsating operating current to said semiconductor light emitting element.
The illuminator preferably further comprises a concave mirror for reflecting said visible light to orient it in a predetermined direction.
Preferably, the emission wavelength of said semiconductor light emitting element is approximately 330 nm.
A read-out device according to the invention comprises: a semiconductor light emitting element for emitting ultraviolet rays; a fluorescent element for absorbing said ultraviolet rays emitted from said semiconductor light emitting element and for releasing light having a longer wavelength than said ultraviolet rays; and a photodetector for detecting said light with the longer wavelength reflected in the exterior, said light emitted released from said fluorescent element being irradiated onto a manuscript to read out information therefrom.
Preferably, said semiconductor light emitting element contains a gallium nitride compound semiconductor in a light emitting layer thereof.
A projector according to the invention for projecting a profile on a translucent medium in an enlarged scale, comprises: a semiconductor light emitting element for emitting ultraviolet rays; a fluorescent element for absorbing said ultraviolet rays emitted from said semiconductor light emitting element and for releasing visible light; and an optical system for collecting said visible light to direct it onto a screen.
Preferably, said semiconductor light emitting element contains a gallium nitride compound semiconductor in a light emitting layer thereof.
A purifier according to the invention comprises: a purifying circuit for passing a liquid or a gas therethrough; and a semiconductor light emitting element located along said purifying circuit to emit ultraviolet rays.
Preferably, said semiconductor light emitting element contains a gallium nitride compound semiconductor in a light emitting layer thereof.
The purifier preferably further comprises an ozone generator along said purifying circuit so that said ultraviolet rays are irradiated to a liquid containing ozone generated by said ozone generator.
The purifier preferably further comprises a heater along said purifying circuit a gas purified by said purifying circuit be discharged after being heated.
Preferably the emission wavelength of said semiconductor light emitting element is approximately 330 nm.
A display device according to the invention comprises: a semiconductor light emitting element for releasing ultraviolet rays; and a display panel having stacked a fluorescent element for absorbing said ultraviolet rays released from said semiconductor light emitting element and for releasing visible light.
Preferably, said semiconductor light emitting element contains a gallium nitride compound semiconductor in a light emitting layer thereof.
Illuminators according to the invention have high mechanical strengths against impulses or vibrations.
Light from the light emitting layer is not extracted directly but converted in wavelength by a fluorescent material. Therefore, it is prevented that the emission wavelength varies with varieties of manufacturing parameters of the semiconductor light emitting elements, drive current, temperature and other inevitable factors. That is, the invention realizes remarkable stability of emission wavelengths and makes it possible to control the emission luminance and the emission wavelength independently.
The fluorescent material may include a plurality of different materials to obtain a plurality of different emission wavelengths. For example, by appropriately mixing different fluorescent materials for red (R), green (G) and blue (B) to form the fluorescent material in each light emitting element, emission of white light can be obtained easily.
The light emitting layer may be made of GaN containing boron. In this case, ultraviolet rays near 330 nm which efficiently excites the fluorescent member can be obtained and enhanced.
Efficiency of the wavelength conversion can be enhanced to more effectively extract the secondary light by reflecting and confining ultraviolet rays emitted from the semiconductor light emitting element and by reflecting and externally guiding the secondary light emitted from the fluorescent member.
The material and the structure of the semiconductor light emitting elements used in a device need not be changed depending on the desired emission wavelength of the device. That is, in conventional techniques, optimum materials had to be selected to form semiconductor light emitting elements in accordance with desired emission wavelengths, such as selecting AlGaAs materials for emission of red light, GaAsP materials for yellow light, InGaAlP or GaP materials for green light and InGaN materials for blue light. However, according to the invention, it is sufficient to select appropriate fluorescent materials, and the material of the semiconductor light emitting element need not be changed.
Even when a device needs an arrangement of a plurality of semiconductor light emitting elements having different emission colors, such elements for different emission colors can be made only by changing the material of the fluorescent member, and all of the semiconductor light emitting elements may be common in materials and structure. This contributes to simplification of the structure of the light emitting device, remarkable reduction of the manufacturing cost and higher reliability. Additionally, by uniforming the drive current, supplied voltage or the size of the elements, its application can be extended remarkably.
As explained above, the invention provides an illuminator and other various kind of applications which are simple in structure, stable in emission wavelength, and capable of highly luminous emission in a wide wavelength range from visible light to infrared bands, and the invention promises great industrial contribution.